Michael E. Barrett

Can we simulate regional groundwater flow in a karst system using equivalent porous media models? Case study, Barton Springs Edwards aquifer, USA

Various approaches can be used to simulate groundwater flow in karst systems, including equivalent porous media distributed parameter, lumped parameter, and dual porosity approaches, as well as discrete fracture or conduit approaches. The purpose of this study was to evaluate two different equivalent porous media approaches: lumped and distributed parameter, for simulating regional groundwater flow in a karst aquifer and to evaluate the adequacy of these approaches. The models were applied to the Barton Springs Edwards aquifer, Texas. Unique aspects of this study include availability of detailed information on recharge from stream-loss studies and on synoptic water levels, long-term continuous water level monitoring in wells throughout the aquifer, and spring discharge data to compare with simulation results. The MODFLOW code was used for the distributed parameter model. Estimation of hydraulic conductivity distribution was optimized by using a combination of trial and error and automated inverse methods. The lumped parameter model consists of five cells representing each of the watersheds contributing recharge to the aquifer. Transient simulations were conducted using both distributed and lumped parameter models for a 10-yr period (1989 ‐ 1998). Both distributed and lumped parameter models fairly accurately simulated the temporal variability in spring discharge; therefore, if the objective of the model is to simulate spring discharge, either distributed or lumped parameter approaches can be used. The distributed parameter model generally reproduced the potentiometric surface at different times. The impact of the amount of pumping on a regional scale on spring discharge can be evaluated using a lumped parameter model; however, more detailed evaluation of the effect of pumping on groundwater levels and spring discharge requires a distributed parameter modeling approach. Sensitivity analyses indicated that spring discharge was much more sensitive to variations in recharge than pumpage, indicating that aquifer management should consider enhanced recharge, in addition to conservation measures, to maintain spring flow. This study shows the ability of equivalent porous media models to simulate regional groundwater flow in a highly karstified aquifer, which is important for water resources and groundwater management. q 2003 Elsevier Science B.V. All rights reserved.

Evaluation of methods for estimating stormwater pollutant loads

This paper investigates a number of methods that can be used to generate constituent concentrations for use in stormwater modeling. These include the use of event mean concentrations (EMCs) and pollutant buildup and washoff formulations. Suspended solids data collected in the Austin, Texas, area from single-land-use watersheds were used to evaluate the usefulness of these methods. Use of a single EMC for all urban land uses was shown to provide a reasonable estimate of solids loads. This suggests that increases in total suspended solids loads resulting from development will be primarily a function of the increase in runoff volume, which in turn may be related to increased impervious cover. Water quality data did not indicate a strong correlation between initial pollutant load on the watershed and length of the antecedent dry period; however, the concentration of suspended solids in stormwater runoff does follow a simple washoff model.

The effect of roofing material on the quality of harvested rainwater

Due to decreases in the availability and quality of traditional water resources, harvested rainwater is increasingly used for potable and non-potable purposes. In this study, we examined the effect of conventional roofing materials (i.e., asphalt fiberglass shingle, Galvalume(®) metal, and concrete tile) and alternative roofing materials (i.e., cool and green) on the quality of harvested rainwater. Results from pilot-scale and full-scale roofs demonstrated that rainwater harvested from any of these roofing materials would require treatment if the consumer wanted to meet United States Environmental Protection Agency primary and secondary drinking water standards or non-potable water reuse guidelines; at a minimum, first-flush diversion, filtration, and disinfection are recommended. Metal roofs are commonly recommended for rainwater harvesting applications, and this study showed that rainwater harvested from metal roofs tends to have lower concentrations of fecal indicator bacteria as compared to other roofing materials. However, concrete tile and cool roofs produced harvested rainwater quality similar to that from the metal roofs, indicating that these roofing materials also are suitable for rainwater harvesting applications. Although the shingle and green roofs produced water quality comparable in many respects to that from the other roofing materials, their dissolved organic carbon concentrations were very high (approximately one order of magnitude higher than what is typical for a finished drinking water in the United States), which might lead to high concentrations of disinfection byproducts after chlorination. Furthermore the concentrations of some metals (e.g., arsenic) in rainwater harvested from the green roof suggest that the quality of commercial growing media should be carefully examined if the harvested rainwater is being considered for domestic use. Hence, roofing material is an important consideration when designing a rainwater catchment.

A parsimonious model for simulating flow in a karst aquifer

Abstract This paper describes the hydrologic system associated with the Barton Springs portion of the Edwards aquifer and presents a lumped parameter model capable of reproducing general historical trends for measured water levels and spring discharge. Recharge to the aquifer was calculated based on flow loss studies of the creeks crossing the recharge zone and on estimates of the rate of diffuse infiltration of rainfall. Flow measurements on each creek above and below the recharge zone were used to develop a relationship between flow above the recharge zone and the rate of recharge. The five-cell groundwater model, each cell corresponding to one of the watersheds of the five main creeks crossing the recharge zone, was developed to support the management objectives of the City of Austin. The model differs from previous models in that the aquifer properties within cells are allowed to vary vertically. Each cell was treated as a tank with an apparent area and the water level of a single well in each cell was used to characterize the conditions in that cell. The simple representation of the hydrologic system produced results comparable to traditional groundwater models with fewer data requirements and calibration parameters. ©1997 Elsevier Science B.V.

An evaluation of geotextiles for temporary sediment control

The performance of geotextiles for sediment control was evaluated in the field and laboratory. Runoff samples collected in the field indicated that essentially no sediment removal was attributable to filtration by the fabric. Silt fences also had little influence on the turbidity of the discharged runoff. Total suspended solids removals of 68 to 90% were found in flume tests in which silt fences were installed. The removal efficiency was correlated with the average detention time of the impounded runoff behind the fence. Flow rates through the fences under field conditions were two orders of magnitude less than would be calculated using standard ASTM index characteristics of the fabrics. This discrepancy resulted from clogging of the fabric with sediment and from the turbulent flow through the fabric openings at the hydraulic heads on the fabrics when used as silt fences. The Center for Research in Water Resources, The Center for Research in Water Resources, The University of Texas at Austin.

STORM WATER POLLUTANT REMOVAL IN ROADSIDE VEGETATED BUFFER STRIPS

A 2-year water quality monitoring project was conducted to evaluate the removal of storm water contaminants by existing vegetated slopes adjacent to freeways. Objectives of the study were to generate design criteria and to determine whether standard roadway design requirements result in buffer strips with treatment equivalent to those specifically engineered for water quality performance. Variables such as width, slope, vegetation density, and hydraulic loading were evaluated by studying the runoff through existing vegetated slopes at four locations in northern California and four locations in southern California. At each location, concrete channels were constructed to capture freeway runoff after it passed through existing vegetated strips of varying widths. The quantity and quality of the runoff discharged from the buffer strip were compared with freeway runoff collected at the edge of pavement. Buffer strips consistently reduced the concentration of suspended solids and total metals in storm water runoff. The strips were also effective in removing dissolved metals when the edge of pavement concentrations were sufficiently high. Little or no change in concentration was observed for nitrogen and phosphorus. Concentrations of organic carbon, dissolved solids, and hardness increased. For the constituents exhibiting a decrease in concentration, steady-state levels were generally achieved within 5 m (16 ft) of the pavement edge for slopes commonly found on highway shoulders and where the vegetation coverage exceeded 80%. Slope, vegetation type and height, highway width, and hydraulic residence time had little or no impact on the discharge concentrations.

Effects of Media and Plant Selection on Biofiltration Performance

AbstractThe goal of this research was to compare the pollutant removal effectiveness of biofiltration systems containing different media and plant species. A laboratory column study was conducted by using three media and two plant species, each with and without a submerged zone intended to promote denitrification. Twenty experiments were run by using synthetic storm water over the course of nine months, and ten of them were analyzed. The three media used were concrete sand, masonry sand, and a medium that meets the City of Austin’s biofiltration specifications. The plant species were Buffalograss 609 and Big Muhly, both commonly found in Texas. The results of this study showed a significant improvement in nutrient removal with the presence of these plants in the filter. The columns without plants were found to export substantial amounts of nitrate/nitrite, whereas the columns with the plants demonstrated a substantial removal of nutrients (59–79% of the total nitrogen and 77–94% of the total phosphorus). ...

Benefits of Porous Asphalt Overlay on Storm Water Quality

This project documents the impact of a porous asphalt overlay on the quality of highway storm water runoff. A porous asphalt overlay, also known as a permeable friction course (PFC) or open-graded friction course, is a layer of porous asphalt, approximately 50 mm thick, that is often applied on top of conventional asphalt on highways to enhance safety and reduce noise. The quality of storm water runoff from a four-lane divided highway in the Austin, Texas, area was monitored before and after the installation of a PFC. The observed concentrations of total suspended solids and pollutants associated with particulate material were much lower in the runoff from the PFC than that derived from the conventional asphalt surface. Concentration reductions were observed for total suspended solids (92%), total lead (91%), total copper (47%), and total zinc (75%). The concentrations of chemical oxygen demand and total Kjeldahl nitrogen were initially lower in the runoff from the PFC but increased abruptly after about 6 months to concentrations that were similar to the concentrations in the runoff from conventional pavement. The concentrations of dissolved constituents were not significantly different between the two pavement types. The observed concentrations of polycyclic aromatic hydrocarbons were below the detection limit for both pavement types.

Methodology for Determining Laboratory and In Situ Hydraulic Conductivity of Asphalt Permeable Friction Course

The permeable friction course (PFC) is a layer of porous asphalt pavement overlain on conventional impervious hot-mix asphalt or portland cement concrete. The drainage properties of PFC are typically considered to be governed primarily by two hydraulic properties: hydraulic conductivity and porosity. Both of these hydraulic properties change over the life cycle of the PFC layer due to clogging of the pore space by sediment. Therefore, determination of the hydraulic conductivity and porosity of PFC can be problematic. Laboratory and particularly field tests are necessary for accurately determining the hydraulic conductivity of the PFC layer. Taking multiple measurements over the life of the pavement shows how these hydraulic characteristics change with time and the varying roadway conditions at which they are evaluated. Constant head laboratory testing has shown that PFC experiences a nonlinear flow relationship as described by the Forchheimer equation. In addition to the laboratory analysis of the hydraulic characteristics, a falling head field test is recommended to determine the in situ hydraulic conductivity. This incorporates the modeling techniques used in the laboratory testing and applies them to the falling head conditions used in the field. The result is a nondestructive test procedure for determining the in situ hydraulic conductivity which is necessary for estimating the extent to which the benefits associated with the drainage characteristics of the PFC layer will persist.

Modeling Sedimentation-Filtration Basins for Urban Watersheds Using Soil and Water Assessment Tool

AbstractSedimentation-filtration (SedFil) basins are one of the storm-water best management practices (BMPs) that are intended to mitigate water quality problems in urban creeks and rivers. A new physically based model of variably saturated flows was developed for simulating flow and sediment in SedFils within the Soil and Water Assessment Tool (SWAT). The integrated SWAT-SedFil model allows for simulation of unsaturated flow in the filtration basin during small storms and fully saturated flow. Unsaturated flow was modeled using a modified Green and Ampt equation, and saturated flow was simulated with Darcy’s Law. Unsaturated flow comprises only a small fraction of large storm events; however, many regular storms are small and may not generate sufficient runoff to create a saturated flow in the filtration basin. Therefore, the combined unsaturated/saturated flow approach for modeling SedFils improved the accuracy of the model, especially in long-term evaluations. The model performs well with respect to es...